The behavioral manifestations and complications of marijuana smoking are probably associated with changes in brain function. With the advent of noninvasive measurement techniques, it is now possible to quantify cerebral blood flow (CBF) and cerebral metabolism (CMR), two indices of brain function in human subjects noninvasively. No information is available on the effects of marijuana on CBF and CMR. The study proposed here is divided into two sections. In Section I, CBF, behavior and mood will be measured before and thrice after smoking high-potency marijuana (2.5% THC), low potency marijuana (1% THC) and a placebo in 48 experienced marijuana smokers (ES and 48 inexperienced marijuana smokers (IS). The high-potency and low- potency marijuana and a placebo will be administered to all subjects and each individual would have had a total of 12 CBF measurements. CBF measurements in this section will be performed with the 133Xenon inhalation technique. The second section involves the measurement of CBF and CMR for oxygen together with behavior and mood before and twice after smoking high-potency (2.5% THC) marijuana (16 ES and 16 IS) or a placebo (8 ES and 8 IS). The measurements will be carried out with Positron Emissin Tomography (PET).